Silicon nanoparticles enable microscopic lasers

URBANA, Ill.  Researchers at the University of Illinois have demonstrated a method of producing fluorescent red, blue, green and yellow nanoparticles from plain-vanilla silicon wafers. The new materials could yield microscopic lasers. They also hold the promise of putting optical communications on electronic chips.

Even though silicon "is the worst material for optics," said Munir Nayfeh, a UI professor of physics, the team devised its synthesis process using "a commercial silicon wafer, the kind used to make ordinary computer chips." When the surface is broken up "into very small particles  on the nanometer scale  the [particles] become highly fluorescent," Nayfeh said.

"We are making sand glow  it's kind of unbelievable, but this is what happens at these nanoscale sizes," said Nayfeh.

Sahraoui Chaieb, a professor of theoretical and applied mechanics, also demonstrated that the fluorescent nanoparticles could be made into microscopic lasers. "Their microlasing is an important step toward a laser-on-a-chip, which could some day replace wires with optical interconnects," said Chaieb, who is also a researcher at the Beckman Institute. Other team members included UI researchers Nicholas Barry and Paul Braun as well as researcher Lubos Mitas at North Carolina State University.

Because silicon is chemically benign within the human body, the new nanoparticles could find immediate applications as fluorescent markers. By attaching biological materials to the nanoparticles, their location could be traced in the body by stimulating their fluorescence with just two photons from harmless infrared beams. Outside the body, the nanoparticles fluoresce when exposed to ultraviolet radiation, and in aggregates they lased in response to a single green mercury lamp.

Snowfall of particles

In Nayfeh's laboratory procedure, which he currently has running on a 12-wafer "assembly line" at the university, standard silicon wafers are gradually immersed in an electrochemical etchant bath  a mix of hydrogen fluoride and hydrogen peroxide, Nayfeh said. An electrical current catalyzes the decimation of the surface of the wafer. "Under this treatment the surface of the wafer gets eroded, and a very fragile network of nanoparticles is formed," he said.

After the surface of the wafer is turned into "Swiss cheese," said Nayfeh, an ultrasonic bath vibrates the wafer, creating a snowfall of nanoparticles. Remarkably, they are confined to a convenient set of discrete sizes completely free of debris.

At that point, "we can easily devise ways to separate them," he said. "We get a 1-nm sphere, which only has 29 silicon atoms  this size is highly fluorescent blue. We also get a 1.67-nm size with about 123 atoms of silicon, which is very bright under UV excitation" and produces green light. A 2.5-nm particle "produces yellow light, and one at 2.9 nm produces red light."

Recycled wafers

According to supercomputer simulations, hydrogen peroxide is the key element that oxidizes the silicon while the electrical current catalyzes the reaction, but only on the surface. The etch bath then eats away the oxidized silicon at the top layer of the wafer only; thus, wafers can be recycled many times through the process.

"The beautiful thing is that you can collect the nanoparticles, store them and reassemble them into any shape or geometry you are interested in," Nayfeh said. "You could make clusters, you could make thin films, you could mix them with gels, you could put them in a liquid to make a laser that communicates underwater." Embedding the silicon nanoparticles in an electronic device would "upgrade it to an optoelectronic device, enabling optical communications between transistors using light as a kind of wire."

While characterizing the nanoparticles, Nayfeh's colleague Chaieb discovered that they can stimulate one another until a higher energy state is achieved, resulting in laser action on a microscopic scale. By assembling the nanoparticles into aggregates as small as 6 microns across, Chaieb was able to demonstrate a microlaser in both the blue and red spectrums.

Sign of hope

"We have seen aggregates of only a few microns that directed both blue beams and even red beams from the larger particles, so it is hopeful that this may be a great gain for lasers," said Nayfeh.

The researchers are currently studying the electronic properties of their nanoparticles in an attempt to create tiny memory modules that can store information on single electrons but that can be read out with optics.

"We want to see if we can store charge on these nanoparticles  they are so tiny that adding just a single electron to one of [them] can raise its potential by almost half a volt," said Nayfeh. It should be possible to build single-electron devices "requiring very little current, producing very little heat and becoming faster and smaller even at room temperature or higher," he said.

An audio recording of reporter R. Colin Johnson's full interview with Munir Nayfeh can be found online at AmpCast.com/RColinJohnson.